[Tilecache] Tilecache - How to get bbox?

[truongpm]

Hi everyone,
I have a wms service (Of VietNam country) as:
 + Query data from MySQL server, using Mapserver.
 + The geoditec in VN2000 (I tried to reprojection it to fix with WGS-84 to
overlay in openstreets map. It worked fine)
 + The coordinator in XY (for examble: -34894054.480356,-20886866.45549...)
I could display this service on top of openstreets map, it worked fine with:
 maxExtent: new OpenLayers.Bounds(-20037508.34, -20037508.34, 20037508.34,
20037508.34)
 map.zoomToExtent(new OpenLayers.Bounds(107.65, 12.56, 109.46,
21.39).transform(map.displayProjection, map.projection));

I get stuck when trying to cache this services by tilecache.
I tried with tilecache configuration as 
 bbox=107.65,12.56,109.46,21.39 (same with Bounds) but i just get only tow
image when test (2 cell of grid i think that). Almost, i get error:"An error
occurred: Current x value 10644925.540000 is too far from tile corner x
10519799.526400
".

I try to solve this problem by searching for 3 day and get some good place
as http://n2.nabble.com/BBOX-of-WMS-requests-td1837186.html
But I don't understand.

So my question are:
#1: is there any theory about the bbox of wms service?
#2: how could i get exactly bbox in my case?

Thanks for your attention, hope you help me. 

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